1. Field of the Invention
The present invention relates to a semitransmissive or reflective electrooptical device having substrates with an electrooptical material held thereby, and an electronic device incorporating such a semitransmissive or reflective electrooptical device. In particular, the present invention is concerned with an IPS (In Plane Switching)-mode active-matrix liquid crystal display device.
The term “semitransmissive liquid crystal display device” referred to herein means a display device having a reflective display area for reflecting incident light from an external source to display an image and a trans-missive display area for transmitting light from a light source disposed behind to display an image.
2. Description of the Related Art
At present, semitransmissive liquid crystal display devices of the IPS type or the VA (Vertical Alignment) type having a wide angle of view have been in widespread use as monitors, and have also been used as television display devices that have increased response characteristics. Liquid crystal display devices are also used on portable information-oriented devices such as cellular phones and digital cameras.
Portable information-oriented devices are mainly used by individuals. Recently, a growing number of portable information-oriented display devices incorporate angularly variable display panels, and should desirably have a wide viewing angle as they are often viewed obliquely. Since display devices for use in portable information-oriented devices are used in a variety of environments ranging from outdoors in sunlight to dark rooms, it is desirable for them to be semitransmissive. Semitransmissive liquid crystal display devices have a reflective display area and a transmissive display area in each pixel.
Heretofore, there have been known a reflective liquid crystal display device wherein a reflective plate reflects incident light from outside the device to provide a display light source, so that the device does not need to have a backlight as a light source, and a transmissive liquid crystal display device having a backlight as a light source.
Since the reflective liquid crystal display device does not require a backlight which is a dispensable component of the transmissive liquid crystal display device, the reflective liquid crystal display device has a low power requirement, a low profile, and a low weight, and is mainly used in portable terminals. The transmissive liquid crystal display device has good visibility even if the ambient light is low since it has a backlight light source.
Japanese patent No. 2955277 (hereinafter referred to as Patent document 1) has proposed a semitransmissive liquid crystal display device having both a reflective area and a transmissive area in each pixel, as a liquid crystal display device that has the advantages of both the reflective liquid crystal display device and the transmissive liquid crystal display device.
The reflective display area reflects incident light from the surrounding area with a reflecting plate to display an image and has a constant contrast ratio irrespective of the surrounding brightness. Therefore, the reflective display area can display good images in relatively bright environments ranging from outdoors in sunlight to rooms. The transmissive display area employs a backlight and has a constant luminance level regardless of the environment. Therefore, the transmissive display area can display images at a high contrast ratio in relatively dark environments ranging from indoors to dark rooms. The semitransmissive liquid crystal display device which has both a reflective display area and a transmissive display area is capable of displaying images at a high contrast ratio in a wide variety of environments ranging from outdoors in sunlight to dark rooms.
The semitransmissive liquid crystal display device has a reflective area and a transmissive area in each pixel electrode. When the ambient light is bright, the backlight is turned off to use the semitransmissive liquid crystal display device as a reflective liquid crystal display device. At this time, the semitransmissive liquid crystal display device has a low power requirement which is a property of the reflective liquid crystal display device. When the ambient light is dark, the backlight is turned on to use the semitransmissive liquid crystal display device as a transmissive liquid crystal display device. At this time, the semitransmissive liquid crystal display device has increased visibility in the surrounding dark which is a property of the transmissive liquid crystal display device.
Liquid crystal display devices are classified into a vertical-electric-field liquid crystal display device in which the molecular axes (referred to as “directors”) of oriented liquid crystal molecules are rotated in a plane perpendicular to the substrate to display an image, and a horizontal-electric-field liquid crystal display device in which the molecular axes are rotated in a plane parallel to the substrate to display an image. Generally, the vertical-electric-field liquid crystal display device has poorer viewing angle characteristics than the horizontal-electric-field liquid crystal display device. It has been expected that a reflective display mode and a transmissive display mode having a wide viewing angle can be simultaneously achieved by applying a horizontal-electric-field drive scheme, such as an IPS system that has been known for having a transmissive display with a wide viewing angle, to a semitransmissive liquid crystal display device.
One example of such an attempt is disclosed as a semitrans-missive IPS system in JP-A No. 2005-338256 (hereinafter referred to as Patent document 2). In recent years, there has been proposed a system wherein the horizontal-electric-field principle is applied to both a reflective area and a transmissive area, as disclosed in JP-A No. 2003-344837 (hereinafter referred to as Patent document 3). The technology disclosed in Patent document 3, which is an example according to the related art in which the horizontal-electric-field principle is applied to both a reflective area and a transmissive area, will be described below with reference to FIG. 1 of the accompanying drawings.
FIG. 1 is a cross-sectional view of a semitransmissive liquid crystal display device in which the horizontal-electric-field principle is applied to both a reflective area and a transmissive area. The semitransmissive liquid crystal display device comprises lower substrate 11, opposite substrate 12, liquid crystal layer 13 sandwiched between substrates 11, 12, and backlight 40 disposed below lower substrate 11. First insulating film 8a is disposed on the surface of lower substrate 11 which faces liquid crystal layer 13.
The semitransmissive liquid crystal display device includes reflective area 5 and transmissive area 6. In reflective area 5, second insulating film 8b is disposed on first insulating film 8a, and reflecting plate 9 is disposed on second insulating film 8b. Third insulating film 8c is disposed on reflecting plate 9, and horizontal-electric-field electrode 7 is disposed on third insulating film 8c. In transmissive area 6, horizontal-electric-field electrode 7 is disposed on first insulating film 8a. 
Reflecting plate 9 is made of metal deposited on second insulating film 8b which has surface irregularities. Reflecting plate 9 reflects incident light diffusely. Third insulating film 8c is disposed on reflecting plate 9 to make the surface of the assembly smooth. In reflective area 5, horizontal-electric-field electrode 7 comprises common electrode 26 and pixel electrode 27, made of ITO, disposed on third insulating film 8c. In transmissive area 6, horizontal-electric-field electrode 7 comprises common electrode 26 and pixel electrode 27, made of ITO, disposed on first insulating film 8a. 
Common electrode 26 and pixel electrode 27 of horizontal-electric-field electrode 7 extend parallel to each other. The liquid crystal molecules of liquid crystal layer 13 are driven by an electric field generated between pixel electrode 27 and common electrode 26. The structure disclosed in Patent document 3 thus simultaneously achieves a reflective display mode and a transmissive display mode with a wide viewing angle.
JP-A No. 2001-337339 (hereinafter referred to as Patent document 4) proposes a liquid crystal display device having a combination of electrode pairs that have different widths and intervals of a pixel electrode layer and a common electrode layer in each pixel. The proposed liquid crystal display device cancels out colorations generated in the respective areas to display images of excellent quality which suffer less colorations due to changes in the viewing direction. A coloration refers to the phenomenon in which the color of a displayed image is seen as blue or red depending on the viewing direction.
FIGS. 2(a) and 2(b) of the accompanying drawings are views showing the pixel structure of the liquid crystal display device disclosed in Patent document 4. FIG. 2(a) is a cross-sectional view of a pixel, and FIG. 2(b) is a plan view of the pixel.
The liquid crystal display device disclosed in Patent document 4 comprises a plurality of pixels each having a common electrode, a plurality of pixel electrodes, and a semiconductor switch device, scanning signal lines, video signal lines for outputting signals to the pixel electrodes. The liquid crystal display device also includes an array substrate that supports the pixels, the scanning signal lines, and the video signal lines on its surface; an opposite substrate disposed in facing relation to the array substrate; and a liquid crystal layer sandwiched between the array substrate and the opposite substrate.
A plurality of electrode pairs, each comprising a common electrode and a pixel electrode, are disposed in each of the pixels, and at least one of the electrode pairs is different in shape from another electrode pair. A certain electrode pair and another electrode pair have differently shaped electrodes which have the same function.
According to the liquid crystal display device disclosed in Patent document 4, one pixel includes a local region in which the shape of an electrode pair is different from the shape of another electrode pair for thereby producing a plurality of different distributions of electric fields. Therefore, the pixel includes a plurality of areas in which the directors of liquid crystal molecules are different from each other. Colorations in those areas cancel out each other to allow the liquid crystal display device to have a wide viewing angle.
Problems of the liquid crystal display devices according to the related art will be described below.
The semitransmissive liquid crystal display device disclosed in Patent document 3 causes a moiré pattern due to the periodic nature of the surface irregularity pattern of the reflecting plate and the electrode pattern, and causes a diffraction (iridescent light) due to light reflected from the electrodes or the reflecting plate.
The reflecting plate in the reflective display area is generally designed to have surface irregularities on the reflecting surface thereof. The minimum distance between the convexities of the surface irregularities is reduced as much as possible in order to scatter more light.
One approach to reduce a moiré pattern and iridescent light would be to randomize surface irregularities of a reflecting film beyond one pixel. Though it is ideal to randomize such surface irregularities, since the surface irregularities need to cover the surface uniformly two-dimensionally for uniformizing the reflected light, it is difficult to eliminate the periodicity of the surface irregularities completely. Therefore, the surface irregularities as they are observed macroscopically have a certain periodic nature which diffracts the reflected light.
In addition, it requires a large expenditure of labor and cost to randomize a mask pattern for a photomask in order to expose a photosensitive organic film. It is not practical to randomize the surface irregularities of the reflecting film unlimitedly beyond one pixel.
The inventor of the present invention has attempted to apply the technology disclosed in Patent document 4 to the pixel structure disclosed in Patent document 3 for reducing the periodicity of the pixel electrodes in one pixel. The term “periodicity” refers to the periodicity of the patterns of the pixel electrodes and the common electrode according to the horizontal-electric-field liquid crystal display device, for example.
However, even if the technology disclosed in Patent document 4 is applied to the pixel structure disclosed in Patent document 3, a moiré pattern is produced, lowering the quality of the displayed image, for the following reasons:
Even if the periodicity of the pixel electrodes in one pixel is eliminated based on the technologies disclosed in Patent document 3 and Patent document 4, since the display device is made of a matrix of pixels, the pixel electrodes are arrayed at the same pitch as the periodic pattern of the pixels. As a result, the display device produces a moiré pattern due to the periodicity of the pixel electrodes and the periodicity of the surface irregularities of the reflecting plate.
If the reflectance of the reflecting plate and the reflectance of the electrodes are different from each other, then since incident light from an external source is diffracted by the electrodes to generate interference fringes and iridescent light, the quality of the displayed images is greatly lowered. Particularly, the pixels disclosed in Patent document 4 are difficult to design for use as pixels for displaying high-definition images because the number of pixel electrodes that are disposed in one pixel is limited.
According to the above liquid crystal display device structure, the surface irregularities of the reflecting plate are arranged in a two-dimensional pattern, and the electrodes are arranged in a one-dimensional pattern. If a two-dimensionally periodic lattice and a one-dimensionally periodic lattice are superposed one on the other, then the combined lattices generate a moiré pattern. Japanese patent No. 2622762 (hereinafter referred to as Patent document 5) discloses a display device for reducing such a moiré pattern.
The display device disclosed in Patent document 5 comprises an image display unit having a plurality of pixels arranged periodically in a two-dimensional pattern and a light control film disposed on the display surface of the image display unit and including a striped pattern of light-permeable stripes and light-impermeable stripes that are alternately arranged at a predetermined pitch for controlling light passing through the light control film.
FIG. 3 of the accompanying drawings is a plan view showing the layout of the light control film with respect to the display surface of the image display unit disclosed in Patent document 5. FIG. 4 of the accompanying drawings is a graph showing the relationship between the angle β formed between the raster scan on the display surface and the stripes of the light control film, and the pitch p of the moiré pattern.
As shown in FIG. 3, the moiré pattern with pitch p is generated by the crossings of the raster scan (straight lines A) and the stripes (straight lines B) which are observed when the image display unit and the light control film are viewed head-on. If the angle β formed between the raster scan and the stripes of the light control film is relatively small, then pitch p of the moiré pattern can be calculated according to the following equation (1):p=|a*k/cos β|/SQRT((tan β)2+(1−k/cos β)2)  (1)
FIG. 4 shows by way of example pitch p of the moiré pattern calculated when β is a variable and k is a parameter in the above equation (1). A review of FIG. 4 indicates that regardless of the magnitude of k, pitch p of the moiré pattern decreases as the angle β increases. If pitch p of the moiré pattern is equal to or smaller than the pitch of the raster scan, then the user is ignorant of the moiré pattern. The angle β for pitch p that is equal to or smaller than the pitch of the raster scan is about β≧3°.
The inventor applied the structure disclosed in Patent document 5 to the pixel structure of the display device disclosed in Patent document 3. However, the displayed image suffered a moiré pattern and had a reduced quality for the reasons described below.
As described above, the distance between the convexities that are closest to each other on the reflecting plate is designed to be as small as possible to cause the reflecting plate to scatter and reflect light efficiently no matter the direction to which external light may be applied. The coordinates at which the convexities are located should ideally be placed at random.
However, if the surface irregularities of the reflecting plate are arranged to cover the surface uniformly two-dimensionally for uniformizing the reflected light, they produce a somewhat periodic structure.
Consequently, in order to reduce a moiré pattern caused by periodic surface irregularities arrayed in a first direction, periodic electrodes may be arrayed in a direction different from the first direction, but a moiré pattern is also produced due to periodic surface irregularities arrayed in a second direction. In other words, the moiré pattern may not necessarily be reduced if the periodic array of the electrodes is rotated over the two-dimensional matrix of surface irregularities.
In addition, if the periodic electrodes are arrayed in a direction different from the direction of the array of periodic surface irregularities, then since the layout of the electric is greatly restricted, the aperture ratio of the pixels (the ratio of an area for reflecting light, which is exclusive of the wiring, to an area representing the entire pixel) will be reduced.
If the distance between adjacent electrodes and the distance between closest convexities are about the same as each other, then the moiré pattern cannot be controlled even by increasing the angle between the direction of the array of periodic electrodes and the direction of the array of periodic surface irregularities.
Moreover, if the electrodes are arranged with the above angle being increased, the proportion of the pixel aperture is reduced, resulting in a reduction in the performance of the display device. Particularly with respect to high-definition display devices, since the size of pixels is small, the number of electrodes and reflecting surface irregularities that can be placed in a pixel are limited, tending to make the electrode pitch and the surface irregularity pitch substantially equal to each other.